BTU Calculator
What size AC do I need for a 500 sq ft room?
About 10,000 BTU per hour. At 20 BTU per sq ft, 500 sq ft x 20 = 10,000 BTU with an 8 ft ceiling, average insulation and sun, 2 people and a moderate climate, so this calculator suggests a 12,000 BTU window or portable unit. In a hot southern climate the same room needs 12,000 BTU, and with heavy sun as well, about 14,000 BTU.
This BTU calculator estimates how many BTU per hour you need to cool or heat one room. Enter the room length and width in feet, pick the ceiling height, and adjust for insulation, sun exposure, number of occupants and climate. You get a rounded BTU figure and a suggested unit size.
It is meant for choosing a window unit, portable AC, ductless mini-split head or space heater for a single room. Alongside the rounded number it shows the exact BTU before rounding, the load in tons, the heat output in watts and the room volume in cubic feet, so you can check it against a product spec sheet. Below the tool you will find ready-made tables for rooms from 150 to 1,500 sq ft and for garages.
Enter your details
How to Use the BTU Calculator
- 1
Choose cooling or heating
Select the Air Conditioning tab to size a cooling unit or the Heating tab to size a heater. Each mode uses a different base rate per square foot.
- 2
Enter room dimensions
Type the room length and width in feet, then choose a ceiling height from 7 to 14+ feet. The calculator multiplies length by width to get floor area.
- 3
Set the adjustment factors
Pick insulation quality, sun exposure, number of occupants (1 to 5+) and your climate zone. Each choice scales the estimate up or down.
- 4
Read your results
Check the rounded BTU per hour, the recommended unit size, tons, watts, exact BTU and cubic feet. Use Clear to reset every field to its default.
How the BTU calculation works
The calculator starts with floor area and a base rate: 20 BTU per square foot for cooling and 35 BTU per square foot for heating. It applies the adjustments below in a fixed order and rounds the final number up to the next 1,000 BTU, since room units are sold in steps like 8,000, 10,000 and 12,000 BTU. If you only know the room in an odd shape, get the floor area first with the square footage calculator.
Ceilings above 8 feet add 2.5% per extra foot. A 7-foot ceiling is treated the same as 8 feet, and the 14+ option is calculated as 14 feet.
BTU = area (sq ft) x base rate (20 cooling, 35 heating) x ceiling factor x insulation factor x sun factor, + 600 for each occupant above 2 (cooling only), then x climate factor, rounded up to the next 1,000- Insulation: poor x1.3, average x1.0, good x0.8, excellent x0.7
- Sun exposure: heavy sun x1.15, average x1.0, shaded x0.9
- Occupants (cooling only): 600 BTU added for each person above two
- Climate, cooling mode: cold x0.8, moderate x1.0, hot x1.2, very hot x1.3
- Climate, heating mode: cold x1.2, moderate x1.0, hot x0.8, very hot x0.7
- Tons = BTU / 12,000 and watts = BTU / 3.412, both from the unrounded BTU
BTU chart by room size (150 to 1,500 sq ft)
These are the numbers this calculator returns with an 8-foot ceiling, average insulation, average sun and 2 occupants. "Hot climate" changes only the climate setting to Hot (Southern US). The last column is the unit size the tool recommends for the moderate-climate cooling figure.
| Room area | Cooling, moderate | Cooling, hot climate | Heating, moderate | Suggested unit |
|---|---|---|---|---|
| 150 sq ft | 3,000 BTU | 4,000 BTU | 6,000 BTU | 5,000 BTU window unit |
| 200 sq ft | 4,000 BTU | 5,000 BTU | 7,000 BTU | 5,000 BTU window unit |
| 250 sq ft | 5,000 BTU | 6,000 BTU | 9,000 BTU | 5,000 BTU window unit |
| 300 sq ft | 6,000 BTU | 8,000 BTU | 11,000 BTU | 8,000 BTU window unit |
| 400 sq ft | 8,000 BTU | 10,000 BTU | 14,000 BTU | 8,000 BTU window unit |
| 500 sq ft | 10,000 BTU | 12,000 BTU | 18,000 BTU | 12,000 BTU window or portable |
| 600 sq ft | 12,000 BTU | 15,000 BTU | 21,000 BTU | 12,000 BTU window or portable |
| 700 sq ft | 14,000 BTU | 17,000 BTU | 25,000 BTU | 18,000 BTU (1.5 ton) split |
| 800 sq ft | 16,000 BTU | 20,000 BTU | 28,000 BTU | 18,000 BTU (1.5 ton) split |
| 1,000 sq ft | 20,000 BTU | 24,000 BTU | 35,000 BTU | 24,000 BTU (2 ton) split |
| 1,200 sq ft | 24,000 BTU | 29,000 BTU | 42,000 BTU | 24,000 BTU (2 ton) split |
| 1,500 sq ft | 30,000 BTU | 36,000 BTU | 53,000 BTU | 36,000 BTU (3 ton) |
Garage AC and heater size
The calculator has no separate garage mode, but you can model a typical uninsulated garage with the inputs it does have: set insulation to Poor and, for cooling, sun exposure to Heavy. The table uses an 8-foot ceiling and 2 occupants. A garage door with no insulation leaks far more than a wall, so treat these as minimums and round up to the next unit size.
| Garage | Cooling, moderate | Cooling, hot climate | Heating, moderate | Heating, cold climate |
|---|---|---|---|---|
| 1-car, 12 x 20 ft (240 sq ft) | 8,000 BTU | 9,000 BTU | 11,000 BTU | 14,000 BTU |
| 2-car, 20 x 20 ft (400 sq ft) | 12,000 BTU | 15,000 BTU | 19,000 BTU | 22,000 BTU |
| 2-car, 22 x 22 ft (484 sq ft) | 15,000 BTU | 18,000 BTU | 23,000 BTU | 27,000 BTU |
| 2.5-car, 24 x 24 ft (576 sq ft) | 18,000 BTU | 21,000 BTU | 27,000 BTU | 32,000 BTU |
Mini-split sizing
Ductless mini-split heads are commonly sold at 9,000, 12,000, 18,000 and 24,000 BTU. Pick the smallest head at or above the rounded BTU this calculator gives for the room it will serve. For example, a 450 sq ft bedroom suite at default settings needs 9,000 BTU, so a 9,000 BTU head fits; a 700 sq ft open living area needs 14,000 BTU, so step up to an 18,000 BTU head. If the same head will also heat the room, run the Heating tab too and size for whichever number is larger, checking the unit's heating capacity at your winter low.
A multi-zone system is sized room by room: run the calculator once per room, then add the results to choose the outdoor unit.
Worked example: a sunny living room in a hot climate
A 15 ft by 20 ft living room with an 8-foot ceiling, average insulation, heavy afternoon sun, four people most evenings, in a hot southern climate:
- Area: 15 x 20 = 300 sq ft, base load 300 x 20 = 6,000 BTU
- Heavy sun: 6,000 x 1.15 = 6,900 BTU
- Occupants: 2 above the baseline, so 6,900 + 1,200 = 8,100 BTU
- Hot climate: 8,100 x 1.2 = 9,720 BTU (the exact BTU shown)
- Rounded up: 10,000 BTU per hour, about 0.81 tons and 2,849 watts of cooling
Edge cases and common mistakes
- Kitchens need extra cooling. The tool has no kitchen setting, so add the commonly used 4,000 BTU yourself.
- The watts figure is heat output expressed in watts, not the electricity the unit draws. Check the label for power use, and use the electricity cost calculator to estimate running cost.
- Choosing 1 person does not lower the estimate. Occupants only add load above two, and have no effect in heating mode.
- Bigger is not better for AC. An oversized unit cools the air before it removes moisture, leaving the room cold and clammy. In a humid basement, pair a right-sized unit with a dehumidifier.
- Poor insulation adds 30% to the load. Improving it with the insulation calculator can let you buy a smaller unit.
When to call an HVAC professional
A rule-of-thumb calculator is fine for one room. It is not the right method for a central air conditioner, heat pump or furnace for a whole house. HVAC contractors use an ACCA Manual J load calculation, which accounts for windows, air leakage, ductwork and local design temperatures room by room. An oversized central system short-cycles and removes less humidity; an undersized one runs nonstop on the hottest and coldest days. Call a pro if you are replacing central equipment, adding ductwork, or the room is above 1,500 sq ft.
Features
- Room size BTU calculation
- Heating and cooling modes
- Insulation factor adjustment
- Climate zone consideration
Frequently Asked Questions
How many BTU do I need per square foot?
This calculator uses 20 BTU per square foot for cooling and 35 BTU per square foot for heating before adjustments. A 300 sq ft room starts at 6,000 BTU for cooling and 10,500 BTU for heating (shown as 11,000 after rounding up). Insulation, sun, occupants, ceiling height and climate then raise or lower that figure.
What size air conditioner do I need for a 12x12 room?
A 12 by 12 ft room is 144 sq ft. At default settings the calculator gives 144 x 20 = 2,880 BTU, rounded up to 3,000 BTU, and recommends a 5,000 BTU window unit since that is about the smallest size sold.
How many square feet does a 10,000 BTU air conditioner cool?
At 20 BTU per square foot, 10,000 BTU covers about 500 sq ft under average conditions in a moderate climate. In a hot climate the factor of 1.2 cuts that to about 417 sq ft, and heavy sun cuts it further.
What size AC do I need for a 2-car garage?
A 20 by 20 ft garage (400 sq ft) with poor insulation and heavy sun needs about 12,000 BTU in a moderate climate and 15,000 BTU in a hot climate, using this calculator. An insulated garage door and walls bring that down noticeably.
What size mini split for a 1,000 sq ft space?
At default settings, 1,000 sq ft needs 20,000 BTU for cooling, so a 24,000 BTU (2 ton) mini-split is the next size up. In a hot climate the estimate is 24,000 BTU, which a 2 ton unit just covers.
Does a higher ceiling need more BTU?
Yes. Each foot above 8 feet adds 2.5%. A 400 sq ft room needs 8,000 BTU with an 8 ft ceiling, 8,400 BTU exact (9,000 rounded) with 10 ft, and 9,200 BTU exact (10,000 rounded) with a 14 ft vaulted ceiling.
Can I use this calculator to size a central AC or furnace?
Only as a rough starting point. Whole-house systems should be sized with an ACCA Manual J load calculation, which a licensed HVAC contractor can run for you. Use this tool for single-room equipment like window units, portable ACs, mini-split heads and space heaters.
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